Return to zero optical signal sources are frequently used in optical communication. The return to zero format has been proven superior to the non return to zero format both in terms of receiver sensitivity and in terms of fiber transmission performance. One particularly useful way to implement a return to zero optical source is to generate a series of optical pulses. The optical pulses may suitably be subjected to modulation using a data signal, in order to use the pulses to carry data.
Various ways of modulating optical pulses exist. For example, the intensity of the pulses may be modulated, or the frequency or phase of the pulses may be modulated. Choices are also available with respect to the stage in the process of pulse generation at which modulation is to take place. Modulation may occur during the process of pulse generation, resulting in the generation of pulses that are modulated or encoded with data, or pulses may be generated that do not carry any data, and may then be subjected to a modulation process so that they will convey information.
Numerous encoding systems exist that may be used to encode or modulate optical signals. One most commonly used modulation technique is on-off keying (OOK), which encodes the binary information by turning an optical pulse on and off to represent digital “1” and “0”, respectively. Another family of techniques that is particularly useful is phase modulation, in which information, such as binary bits, is encoded through the phase difference between succeeding pulses. Various modulation systems may be used, including differential phase shift keying (DPSK), of which
      π    2    -  DPSKand minimum shift keying (MSK) are variants.
      π    2    -  DPSKand MSK systems represent bits by phase differences of
            π      2        ⁢                  ⁢    and    ⁢          -      π    2  between adjacent pulses. For example, a “0” may be represented by a phase difference of
  π  2and a “1” may be represented by a phase difference of
  -            π      2        .  
An additional technique that has been studied primarily in connection with OOK modulation is vestigial sideband (VSB) filtering. VSB filtering employs an optical filter with the center of its passband detuned either up or down by a predetermined amount in order to filter the optical signal and thereby reduce its bandwidth. The amount of detuning may suitably be approximately 10 GHz in cases where the desired bit rate is 40 Gb/s.
It has also been found that
            π      2        -    DPSK    ,including MSK, is analogous to DPSK with VSB filtering. Vestigial sideband filtering has drawn considerable interest, but it is difficult to implement under real world conditions. The reason for this difficulty is that the use of vestigial sideband filtering according to prior art techniques has required precise optical filtering. The filter center frequency, or wavelength, and the filter shape used in prior art implementations of VSB, are critical and difficult to achieve reliably. Therefore, in the prior art, VSB has frequently proven to be an interesting but impractical technique.
It is often convenient to employ electrically controlled devices for generating optical signals such as pulse streams. Such devices receive an optical signal as an input, as well as one or more electrical signals to influence the optical signal in order to produce an optical output having desired characteristics. The use of electrical control signals often provides a relatively simple and inexpensive way to produce the desired characteristics. Optical techniques, such as optical filtering, are frequently more complex and more difficult to control and more costly than electrical techniques.
There exists, therefore, a need for systems and techniques for producing an optical pulse stream using electrical control methods to provide desired characteristics of the pulse stream, with the pulse stream exhibiting desired phase characteristics, including phase characteristics that allow for the use of the pulse stream in both OOK and DPSK systems to achieve desired effects similar to that of optical vestigial sideband filtering.